The present invention relates to determination of a property of an optical device, e.g. the group-delay of the optical device.
In modern optical communication systems, the duration of information-carrying optical pulses is becoming increasingly short. In 40 Gb/s communication systems, the data pulses are shorter than 25 ps. Therefore, it is becoming increasingly important to measure e.g. the group-delay of optical devices with an accuracy of better than 1 ps. E.g. a heterodyne optical network analyzer has the potential to make such extremely precise and accurate measurements of a property of an optical device.
Therefore, it is an object of the invention to provide improved determination of a property of an optical device, in particular to improved determination of the group-delay of an optical device.
The object is solved by the independent claims.
Heterodyne optical network analyzers are for example used for measurements of group delay in optical components. Typically in heterodyne optical network analyzers or analyzer systems, two interferometers are involved. A tunable laser source (TLS) launches light into the two interferometers, and this light is continuously tuned from a start-frequency to a stop-frequency. One of the interferometers, the device-under-test (DUT) interferometer, measures the group delay of a DUT as a function of frequency. In order to measure this group delay, precise knowledge of the frequency tuning as a function of time is necessary. This necessary information about the time-dependence of the frequency-tuning is supplied from measurements made with the second interferometer, the reference interferometer. However, two practically unavoidable characteristics of such systems interact to create a significant limitation on the measurement precision of these devices. These two problems are nonlinear sweep of the tunable laser source and length mismatches present in the system.
For example, a problem can arise if an extra length of fiber is present before the reference interferometer and not the DUT interferometer. In this situation, an error occurs in measuring the group delay of the DUT because the group delay measurement relies on frequency-tuning rates measured with the reference interferometer. The rates measured by the reference interferometer are, because of the extra fiber length, delayed in time with respect to the actual frequency-tuning rates appropriate in the DUT interferometer. The same issue arises when the electronic delays of the photo receivers that measure the optical heterodyne signals are not identical. This situation is equivalent to a longer path leading to or from one of the interferometers than the other. In the present application the above is called an external time-delay.
A second problem can arise if the two interferometers are not symmetrical, i.e. if the free spectral range of the interferometers is different. The free-spectral range is inversely proportional to the difference in length between the two arms of an interferometer. In almost all heterodyne optical network analyzers, the two arms of an interferometer have different lengths. If the difference is not the same in both interferometers, the time-dependence of the frequency-tuning measured by the reference interferometer will not correctly describe the time-dependence of the frequency-tuning used to measure the group delay in the DUT interferometer. This type of length mismatch is unavoidable if one wishes to use the same optical setup to measure several different DUT with differing lengths. In the present application the above is called an internal time-delay.
With other words: The reason for the time delay can be internal, i.e. the reason for the delay lies within the measurement device, e.g. within one of the arms of an interferometer, used to measure the optical property, and can be external, i.e. the reason for the delay lies not within the measurement device, e.g. within one of the arms of an interferometer, used to measure the optical property, but occurs on the way of the light before entering the measurement device or after having left to measurement device.
These length mismatches become particularly detrimental when the TLS does not tune its frequency linearly. Nonlinear frequency tuning causes significant errors in the measurement when length mismatches are present.
The present invention proposes a time-delay, applied in hardware or software, to correct for the length mismatches, electronic time-delays, and nonlinear frequency tuning that ordinarily limit the accuracy of the measurement of the optical property. An advantage of the present invention is therefore improved determination of the group-delay of an optical device by applying a time-delay shift to compensate for internal and/or external time-delays to compensate e.g. for group delay errors induced by the interaction of internal and/or external time-delays caused by internal and/or external length mismatches, external time-delays caused by electronic time-delays and local oscillator nonlinear frequency sweep.
The time-delay can be derived theoretically from the setup of the analyzer system, e.g. mathematically, or can be derived in an empiric way, e.g. by testing several time-delays which are supposed to suit to an used analyzer system and by sorting out the time-delay giving the best results for this system.
The time-delay can be a constant value or at least be sufficiently approximated thereby. However, the time delay may also be dependent e.g. on the wavelength or might vary overtime. In such case the time-delay can be varied dynamically or adaptively to the system. However, in case the dependency of the time-delay (e.g. on time or wavelength) is sufficiently small, a static time-delay might be sufficient to compensate for the inherent time-delay of the system.
The time-delay can be introduced in hardware preferably by applying an electronic delay of the appropriate amount in the receiver system electronics of a heterodyne optical network analyzer. Alternatively, an extra length of fiber could preferably be applied to the path leading to one of the detectors. In software, a delay can preferably be applied numerically to the measured signals.
In conclusion, according to the present invention a time-delay, applied in hardware or software, can correct errors due to various length and electronic delay mismatches in the system and the interaction of these mismatches with nonlinear tuning of the TLS and therefore can ultimately enable the full measurement capabilities of a heterodyne optical network analyzer.
In the inventive apparatus the beam splitters can be couplers, also. The numbering of the beam splitters, i.e. first, second, third . . . beam splitter, does not necessarily imply that the splitters have to be different. Actually all or some of them can be the same, e.g. when used in an interferometer setup of Michelson type.
Other preferred embodiments are shown by the dependent claims.
It is clear that the invention can be partly embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit.